Combustion products and effluents resulting from the burning of fossil fuels due to either natural causes or from a variety of man-made devices contain various by-products. The majority of these by-products have been identified and classified by environmental regulatory agencies around the world as major sources of air pollution. Pollutants of particular concern found in the combustion products are NO and NO2 (collectively referred to as NOx or nitrogen oxides).
Various post-combustion methods have been developed and have been applied to reduce the emissions of NOx. Of these methods, Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) are the most commonly practiced. However, both methods require the addition of a reducing agent(s) to assist in the conversion of NOx into benign compounds. Presently, the most commonly used reducing agent is ammonia (NH3) in any of its many forms. Unless specified in this specification, ammonia refers generally to any form, including but not limited to ammonia and aqua ammonia.
The reduction of NOx into benign compounds is depicted in the following reactions:NO+½ O2→NO2 6 NO2+8 NH3→7 N2+12 H2O
Ammonia is considered to be a toxic gas by environmental regulatory agencies. As the use of ammonia has become more widespread, a concern exists regarding the possibility of accidental leakage into the atmosphere, and as a result, new regulations have been implemented to limit the use of anhydrous ammonia (particularly in populated areas) and require that it be substituted with ammonium hydroxide (NH4OH), commonly known as aqua ammonia or aqueous ammonia. Ammonium hydroxide refers to a solution of ammonia in water.
Ammonia concentration in ammonium hydroxide is standardized commercially at two levels, namely, about 29.4% and about 19% NH3 by weight, referred to as 19% NH4OH or 29.4% NH4OH. The boiling point of the 29.4% NH4OH solution is about 80° F. while the boiling point of the 19% NH4OH solution is about 118° F. Due to its relatively high boiling point temperature, the 19% by weight aqua ammonia has been preferred by U.S. regulatory agencies since it reduces the potential for accidental escape or release of ammonia gas into the atmosphere.
To ensure maximum uniformity of distribution and mixing of ammonia with the NOx in the flue gas stream, a boosting or carrying fluid is required in order to achieve the desired dispersion into the flue gas stream. The amount of carrying fluid used varies with specific application. The objective is to increase injection jet momentum and therefore, enhance mixing. Ambient air or a portion of the hot flue gas provided by the combustion equipment itself has commonly been used as the carrying fluid.
In the prior art system, ammonia is vaporized through the use of a vaporizer where the heat of vaporization is provided entirely by the sensible heat of the carrying fluid. In most cases, the amount of carrying fluid used to assist in the dispersion of the ammonia vapor could be several times the theoretical amount needed to heat and vaporize the ammonia.
When ambient air is used as the carrying fluid of choice, the preferred method to heat the air is by means of an electric heater. The total energy demand to heat the carrying fluid includes the heat of vaporization of the ammonia plus the sensible heat gained by the carrying fluid from ambient temperature to a final mixture temperature. The final mixture temperature must be kept above the dew point temperature of the mixture in order to avoid re-absorption of the ammonia vapor into the condensate.
A limitation of the prior art involves using the sensible heat of the carrying fluid to vaporize the ammonia. Since the heat capacity of a carrying fluid is directly related to the amount of carrying fluid required, two options are available: 1) increase the initial temperature of the carrying fluid to a level such that the final preheated air temperature prior to the introduction of ammonia would be high enough to provide the necessary temperature to vaporize the ammonia in a relatively short time, and/or 2) if the initial temperature of the carrying fluid cannot be raised sufficiently, increase the mass flow of the carrying fluid. Neither of these options is without disadvantages. Increasing the final temperature of the carrying fluid is costly since it requires the use of a larger and more costly electric heater. Increasing the mass of the carrying fluid has an adverse impact on the system by creating an increased pressure loss through both the electric heater and the vaporizer itself. Further, increasing the mass of the carrying fluid to aid in dispersing the reducing agent may also increase the costs of the system. If an electric heater is used to heat the carrying fluid, the size of the heater, in terms of both physical size and watt capacity, has to be increased accordingly in order to maintain the proper temperature for vaporization of the ammonia. Increasing the size of the heater results in a more expensive heater, including the control system and hardware, and also results in an increase in operating costs. Further, electric heaters are generally structured in a number of bundles of heating elements. A larger capacity heater normally requires more heating elements which in turn causes a higher pressure loss across the heater. In addition, increasing the mass of the carrying fluid requires a larger blower which itself is more costly and also requires more power to operate to satisfy the increase in pressure loss through the electric heater as well as the high volume of the carrying fluid. Therefore, a method is needed to provide for the vaporization of ammonia without any of the aforementioned drawbacks.